The invention is in the field of inverter circuits employing switches, such as transistors, for producing a-c output energy from d-c input energy.
Inverter circuits generally are classified in the form of three known circuit configurations: full bridge, half bridge, and push pull; and employ one or more switching devices, such as transistors, connected to alternately and repetitively apply electrical energy in a-c form to a load such as a motor, induction heating device, or one or more electric lamps. The inverter circuit is intended to deliver to the load an a-c waveform that is symmetrical, i.e. which deviates to equal plus or minus voltage and current values from its zero or median reference value and which has approximately equal time periods of plus and minus voltage values during its repetitive cycles. If the a-c waveform delivered to the load should become non-symmetrical, even for a short time, the load might become electrically or magnetically saturated (especially if the load is inductive or if it includes a transformer), which can cause a failure in the inverter circuit such as shorting of transistors or shorting or burning out of transformer windings. Such a non-symmetry of the inverter waveform can be caused by a premature switching of an inverter transistor due to extraneous or transient voltage pulses in the circuit, such as can be caused by power line voltage transients or by radiation from nearby electrical appliances or equipment.
The above-referenced patent application in directed to an inverter circuit of the half bridge type, and the present invention will specifically be described with respect to an improved control oscillator having symmetry control for such an inverter circuit.